ABSTRACT ! Bacteria use cyclic-di-GMP (c-di-GMP) as a secondary signaling molecule to relay environmental cues to phenotypic changes, including biofilm formation and virulence. C-di-GMP is degraded in two sequential enzymatic steps: 1. Linearization to pGpG by phosphodiesterase-A (PDE-A) and 2. Hydrolysis of pGpG to GMP by PDE-B. While PDE-A enzymes have been studied more extensively, we only recently identified oligoribonuclase (Orn) as the primary PDE-B in Pseudomonas aeruginosa. A P. aeruginosa ?orn mutant has elevated levels of pGpG and c-di-GMP, resulting in hyperbiofilm formation. Notably, our preliminary data indicate that the ?orn mutant is also unable to disseminate in a murine model of catheter-associated urinary tract infection (CAUTI). However, the underlying molecular mechanisms remain enigmatic. Orn is described as a 3' to 5' exoribonuclease that is thought to cleave short oligonucleotides from 2-7 residues in length, completing RNA degradation. Orn is unique amongst exoribonucleases since it is the only member known to be essential in many gammaproteobacteria. Based on our data, Orn appears to function at the intersection between RNA degradation and c-di-GMP signaling, but how an enzyme regulates both dinucleotide signaling and global RNA pools is poorly understood. To bridge this fundamental knowledge gap regarding Orn's unique function, we developed an interdisciplinary research plan. We hypothesize that Orn's enzymatic and physiological function deviates from the popular view that Orn acts as rather unspecific nano-RNase. Instead, based on new structures of Orn in complex with pGpG that reveal a constrained catalytic site optimized for dinucleotide substrates, we propose that Orn functions primarily as an endonuclease for dinucleotides. Further preliminary data demonstrate that organisms that do not encode orn have additional genes that function as PDE-B. Our previous publications and preliminary data form the scientific premise underlying our overarching hypothesis that PDE-Bs are dinucleotidases that regulate c-di-GMP signaling and chronic infection. To test our hypothesis, we will complete the following aims: 1. Characterize the molecular basis for substrate recognition and catalysis by PDE-Bs; 2. Elucidate Orn substrate preferences and effects on oligonucleotide pools; and 3. Determine the pathways regulated by Orn during chronic P. aeruginosa catheter-associated urinary tract infections. Results from our proposed studies will reveal the structural basis for PDE-Bs preference for dinucleotides, provide biochemical evidence that PDE-Bs are dinucleotidases, reveal the impact of loss of PDE-Bs on the accumulation of diucleotide and oligonucleotide pools, and uncover altered regulation leading to defects during chronic infections. The impact of the grant is to understand the intersection of RNA degradation and cyclic dinucleotide signaling and to assign a defined function for PDE-Bs that is consistent with their observed physiological roles in signaling and infection.